In an exposure process in the manufacture of devices such as a semiconductor device, a pattern formed on a mask such as a photomask or reticle is projected and transferred onto a substrate such as a wafer coated with a resist to perform pattern formation. At this time, if a foreign matter is present on the mask, the foreign matter is transferred onto the substrate together with the pattern, thereby causing defects. To prevent this, a pattern protective film or pattern protective plate called a pellicle is attached to the mask. Examples of the pellicle can include a membranous substance made of a synthetic resin, a plate-like substance made of, e.g., quartz, and the like. The pellicle is supported by a pellicle support frame at a position offset from the pattern surface of the mask by a predetermined distance. The use of the pellicle causes the foreign matter to attach to a pellicle surface offset from the pattern surface of the mask by the predetermined distance. For this reason, the foreign matter is out of focus on the surface of the substrate and only makes the illuminance of illumination light nonuniform. Consequently, the use of the pellicle reduces an effect of the foreign matter in exposure.
FIG. 1 is a schematic view showing the structure of a mask with a pellicle. A pellicle structure 24 is generally attached to the pattern surface side of a mask 23 through a pellicle support frame 25 with an adhesive or the like. The pellicle structure 24 comprises the pellicle support frame 25 arranged to surround a pattern formed on the mask 23, and a pellicle film (or pellicle plate; in this specification, a pellicle film or pellicle plate is generically called a pellicle) 26 which is attached to one end face of the pellicle support frame 25 and has high transmittance with respect to exposure light. When a space (to be referred to as a pellicle space) enclosed with the pellicle structure 24 and mask 23 is completely sealed, the pellicle 26 may expand or contract. To solve this problem, a vent hole 27 is provided in the pellicle support frame 25 so as to prevent occurrence of a difference in atmospheric pressure between the inside and outside of the pellicle space. A dustproof filter (not shown) is also provided in the vent hole 27 or its entrance or exit to prevent foreign matter from entering the pellicle space from the vent hole 27.
A manufacturing process of a semiconductor device formed with an ultramicropattern such as an LSI or VLSI employs a reduction projection exposure apparatus which prints by reduction projection a circuit pattern formed on a photomask onto a wafer coated with a resist. Along with an increase in packing density of semiconductor elements, demands for further micropatterning of a circuit pattern have arisen. As a resist process develops, further miniaturization of the exposure line width has been demanded for an exposure apparatus.
To increase the resolving power of the exposure apparatus, there is available a method of decreasing the exposure wavelength and a method of increasing the numerical aperture (NA) of a projection optical system. As for a decrease in the exposure wavelength, the 365-nm i-line has been replaced by a KrF excimer laser with an oscillation wavelength around 248 nm and has further been replaced by an ArF excimer laser with an oscillation wavelength of around 193 nm. A fluorine (F2) excimer laser with an oscillation wavelength around 157 nm is also under development as the next-generation light source.
Far ultraviolet rays, particularly, an ArF excimer laser with a wavelength around 193 nm, and a fluorine (F2) excimer laser with an oscillation wavelength around 157 nm are known to have a plurality of oxygen (O2) absorption bands around their wavelength bands.
For example, fluorine excimer laser light has been applied to an exposure apparatus because of a short wavelength of 157 nm. The 157-nm wavelength falls within a wavelength region generally called a vacuum ultraviolet region. In this wavelength region, light is greatly absorbed by oxygen molecules, and hardly passes through the air. The fluorine excimer laser can only be applied in an environment in which the atmospheric pressure is decreased to almost vacuum and the oxygen concentration is sufficiently decreased.
Oxygen absorbs light to generate ozone (O3), and the ozone promotes absorption of light, greatly decreasing the transmittance. In addition, various products generated by ozone are deposited on the surface of an optical element, decreasing the efficiency of the optical system.
To prevent this, a purge mechanism using inert gas such as nitrogen suppresses to a low level of several ppm order or less the oxygen concentration in the optical path of the exposure optical system of a projection exposure apparatus using far ultraviolet rays, particularly, an ArF excimer laser with a wavelength around 193 nm or a fluorine (F2) excimer laser with an oscillation wavelength around 157 nm as a light source. This also applies to moisture, which must be removed to the ppm order or less.
A large quantity of inert gas is required to suppress the oxygen concentration and moisture concentration throughout the entire exposure apparatus to a level of several ppm or less by a purge mechanism using inert gas, thus posing a problem in terms of the apparatus operating cost. Purge must be performed until the oxygen concentration and moisture concentration are reduced to several ppm order or less, particularly, for a portion serving as the optical path of ultraviolet light. On the other hand, the oxygen concentration and moisture concentration are typically assumed to be at about 100 to 1,000 ppm in an area except an exposure optical path, i.e., a reticle or wafer transport area.
A load-lock mechanism is arranged at a coupling portion between the inside and outside of a purge space such as the interior of an exposure apparatus. When a reticle or wafer is to be externally loaded, the interior of the load-lock mechanism is temporarily shielded from outside air. After the impurity in the load-lock mechanism is purged with inert gas, the reticle or wafer is loaded into the exposure apparatus.
FIG. 2 is a schematic view showing an example of a semiconductor exposure apparatus having a fluorine (F2) excimer laser as a light source and a load-lock mechanism. In FIG. 2, reference numeral 1 denotes a reticle stage (mask stage) on which a reticle (mask) bearing a pattern is mounted; and 2, an exposure section including a projection optical system which projects the pattern on the reticle onto a wafer, an illumination optical system for illuminating the reticle with illumination light, and the like. The illumination light is guided from a fluorine (F2) excimer laser light source (not shown) to the exposure section by an optical guide system.
Reference numeral 8 denotes a housing which covers the exposure optical axis around the reticle stage 1 and whose interior is purged with inert gas; 3, an environment chamber which covers the entire exposure apparatus and manages its environment by circulating air controlled to a predetermined temperature and whose internal temperature is kept constant; 4, an air-conditioner which supplies temperature-controlled clean air to the chamber 3 and keeps a predetermined block such as an optical system in an inert gas atmosphere; and 13, a reticle load-lock used to load a reticle into the housing 8.
Reference numeral 100 denotes a reticle transport apparatus for transporting a reticle, which comprises a reticle hand (transport hand) 15 which holds the reticle and a driving mechanism (e.g., a SCARA robot). Reference numeral 18 denotes a reticle stocker which stocks a plurality of reticles in the housing 8; 22, a foreign matter inspection device which measures the size and number of foreign matters such as dust deposited on a reticle surface or pellicle surface; and 20, an SMIF (Standard Mechanical Interface) pod which accommodates one or a plurality of reticles. Reference numeral 200 denotes a reticle transport apparatus which transports a reticle between the SMIF pod 20 and the load-lock 13 and comprises a reticle hand (transport hand) 16 which holds the reticle and a driving mechanism (e.g., a SCARA robot) which drives the reticle hand 16.
A reticle is loaded into the load-lock 13, and the interior of the load-lock 13 is purged with inert gas until the interior reaches an inert gas atmosphere similar to that in the housing 8. After that, the reticle is transported by the reticle hand 15 to the reticle stage 1, reticle stocker 18, or foreign matter inspection device 22.
As described above, an exposure apparatus using ultraviolet rays, particularly, an ArF excimer laser or a fluorine (F2) excimer laser suffers from large absorption of the ArF excimer laser or fluorine (F2) excimer laser of its wavelength by oxygen and moisture. To obtain a sufficient transmittance and stability of ultraviolet rays, the oxygen and moisture concentrations must be reduced and controlled strictly. For this purpose, a load-lock mechanism is arranged at a coupling portion between the inside and outside of the exposure apparatus. When a reticle or wafer is to be externally loaded into the exposure apparatus, the load-lock mechanism is temporarily shielded from outside air. After the impurity in the load-lock mechanism is purged with inert gas, the reticle or wafer is loaded into the exposure apparatus.
To ensure the transmittance and stability of the fluorine (F2) excimer laser, the entire reticle stage (wafer stage) including the end face of a projection lens and a measurement interference optical system is housed in an airtight chamber, and the interior of the chamber is purged with high-purity inert gas. In addition, the load-lock chamber is disposed adjacent to the airtight chamber in order to load/unload a wafer or reticle into/from the airtight chamber while maintaining a constant internal inert gas concentration. A reticle loaded into the load-lock chamber bears a pellicle, and a pellicle space can communicate with outside air only through a relatively small vent hole. This structure prolongs a time required to complete purge in the pellicle space even after the interior of the load-lock chamber reaches a predetermined inert gas concentration, degrading the productivity. Also, when a valve or dustproof filter is arranged in a path including a vent hole, the ventilation resistance increases, and the purge time is further prolonged.
To increase the gas purge efficiency in a pellicle space, there is proposed a method of actively purging a pellicle with gas in, e.g., Japanese Patent Laid-Open Nos. 2001-133960 and 2001-133961. A gas purge station which actively purges the pellicle space with gas is preferably provided in at least one place such as the load-lock 13, reticle stocker 18, or the like, shown in FIG. 2.
Assume that a vent hole is formed in a pellicle support frame, and a pellicle space is temporarily filled and sealed with inert gas of ppm order. If the oxygen concentration in a space where a reticle with a pellicle is set while being transported into a reticle stocker or while being stocked in the reticle stocker after exposure is higher than that in the pellicle space, oxygen enters the pellicle space through a vent hole. Thus, it is very difficult to maintain the oxygen concentration in the ppm order. If the reticle with the pellicle is temporarily transported outside a load-lock and is reloaded into the load-lock, the oxygen concentration of the atmosphere in the pellicle space becomes a % order, and a long time is required to purge the pellicle space again.
If a vent hole is completely sealed (e.g., if the vent hole formed in advance in a pellicle support frame is sealed after injecting inert gas into a pellicle space through the vent hole), the pellicle space forms a completely closed space. Accordingly, a pellicle film may deflect, expand, or contract due to the difference in atmospheric pressure or oxygen concentration between the inside and outside of the pellicle space.
If a pellicle film deflects in an exposure apparatus, a problem may occur. More specifically, the foreign matter inspection device 22 cannot precisely detect any foreign matter.
If a projection exposure apparatus using far ultraviolet rays such as a fluorine (F2) excimer laser employs a pellicle film made of, e.g., a fluorine-based resin which is conventionally used in KrF exposure or ArF exposure, the durability of the pellicle film becomes a problem because its thickness decreases due to photodecomposition of the film material. For this reason, a pellicle made of glass having higher durability and a thickness of about 0.3 mm to 0.8 mm has been proposed instead. If a glass pellicle in an exposure optical path deflects, a problem such as a change in size of a transfer pattern may occur. This may adversely affect the exposure performance.
In Japanese Patent Laid-Open No. 2001-500669, there is proposed a process of purging an SMIF pod with inert gas to control the humidity and contents of oxygen and particulate matter (foreign matter) to a low level. Although the proposal is intended for a wafer SMIF pod, an SMIF pod which accommodates a reticle is also preferably purged with inert gas. This is because if sulfuric acid, ammonia, or the like, is present on a chromium surface (a circuit pattern is made from chromium) or glass surface of the reticle, it may react with oxygen contained in air due to exposure energy to cause fogging and may pass through a pellicle. In this case, in FIG. 2, a reticle transport area (transport space) between the SMIF pod 20 and the load-lock 13 is not purged with gas, and oxygen enters a pellicle space through a vent hole formed in a pellicle support frame during the transport of the reticle by the reticle hand 16 due to the difference in atmospheric pressure or oxygen concentration between the inside and outside of the pellicle space. This changes the oxygen concentration in the pellicle space to the % order. Under these circumstances, the pellicle space needs to be purged again, and thus a long purge time is required, which is inefficient.